div {
    margin: 2em;
}

.div1 {
    width: 200px;
    height: 200px;
    border-radius: 100px;
    background: #fb3;
}

.div2 {
    width: 200px;
    height: 100px;
    border-radius: 50%/50%;
    background: #fb3;
}

.div3 {
    width: 200px;
    height: 100px;
    border-radius: 50%/100% 100% 0 0;
    background: #fb3;
}

.div4 {
    width: 200px;
    height: 100px;
    border-radius: 100% 0 0 0;
    background: #fb3;
}